poor running 911
 

84 911 runs poor after dead battery jumped and then replaced. Starts and idles fine. Won't rev, give it gas and it stumbles very badly and backfires. Can't drive it at all as it has no power. Pumping gas and it will rev some but still very rough, backfires, and then stumbles to idle.
DMC resistances check out. pump relay replaced along with fuse. New cap (had a crack), rotor, coil, and spark plug wires replaced and it runs a little better, not much though. Platinum plugs cleaned and re-gaped. Reference sensor and speed sensor resistance check out and car won't start with either of them unplugged. Oxygen sensor replaced since it's near 60,000 miles. Also, fuel pump spinning fine and running pressure is good.
Could this still be the speed sensor? Any thoughts would be appreciated.

Make sure that your firing order is correct. Could be the spark plug wires were put back in the wrong order. (Which would be the back fire problem.)

Did you by chance put the reference sensor/speed sensor in the wrong order as well when checked? Just a guess on my part.

I am no expert, but check all the fundamental stuff to ensure you don't start going down the wrong path. Like for example, were the valves set during this process? Did the distributor get moved in the wrong direction when setting the timing? (Like forgetting to ensure the bolt is tight on the distributor, so it won't move during running condition.)

Also, in view of the battery being jumped to start the car, check all your fuses to ensure none are blown.

Oh, I forgot to mention this earlier: I personally don't use Platinum plugs because they were really terrible for my car, so I went back to regular copper Bosch plugs.

I experienced the exact same problem you describe here by doing a tune-up maintenance with Platinum plugs. Just my experience with them and not an expert opinion on what to use; just an observation on my part and a personal preference.

Good luck!SmileWavy

Sounds like a huge vacuum leak. Do you have a pop-off valve? Did the low battery cause a backfire? Is the pop-off valve still sealed?

If you've JUST changed the wires, check the firing order first.....an easy mistake. Good luck

Cylinder head temp sensor?

Thanks for all the ideas so far. The plug wire order is OK and the problem started before the wires were replaced. Platinum plugs have been in for several thousand miles and have run good in the past. By the way Bruce Anderson at his tune up class also recommended copper plugs for this year 911. Timing hasn't been changed since it was running good. I haven't checked the head temp sensor yet but will do that soon.

Oh, I forgot to mention that 84 doesn't have a pop-off valve and the trouble started before the backfiring which is pretty minor by the way.

Any solution to this problem. I hadn't made any changes. Ran great yesterday on a 200 mile trip to Big Sur. Today I started experiencing same problems as described.

Still runs rough at any rpm higher than idle. Idles fine. Have since replaced the cylinder head temperature sensor, camshaft sensor, and speed sensor. All the work so far has made a little improvement but not much. I'll be looking into a air leak next. I've done some looking and there is no obvious leak. Let me know what you find with your car. Good luck.

It's possible that the ECU for the Motronics has failed after your battery problem.

la975,

before you start sending off your ECU for diagnosis, you may want to do a quick check for vacuum leak.

You've already stated that your motor starts and idles fine. With a warm running engine, remove the oil filler cap. If your idle does not immediately stutter and idle roughly with the cap off, you probably have an intake leak somewhere.

Another test is to spray carb cleaner or ether around all intake ports and vacuum hoses while the engine is running. Any hesitation in idle while spraying indicates an intake leak.

I've heard that the intake manifold gaskets get "sucked in" as they age and crack, and are sometimes difficult to pin down when leaking.

If, after all this and you've found no intake leak, your problem will most likely be somewhere in the ignition system.

Have you checked your voltage? Low voltage can make the ECU run wonky.

If you read the first post you will see that this all started after a battery problem where the owner jumped the car possibly causing a surge. Unlikely a vacuum leak showed up at the moment he restarted the car.

yeah, true...but he stated that he wanted to hunt for any leaks, and a vacuum check only takes a couple of minutes, and doesn't hurt anything. Anything can happen...better safe than sorry. :D

LA975,

My car has been in the shop since yesterday and he's scratching his head over it. Yes, he is a Porsche mechanic. I just want me baby back.

I really have a hunch we have the same problem. I'll keep you posted.

Why don't you try a new ignition coil?

Wow, this thread is very messy.

To the OP, can you summarize the parts that you've checked and/or replaced? If it were my car, I'd probably start at the beginning and make sure the battery is fully charged, clean and tight contacts, make sure the dme relay is good or replace with a known good one. Clean the dme ground wires until they are shiny and make sure they are tight. Check for at least 12v to 12.5v at the dme harness or at the ICV harness. Make sure your alternator is working by checking the voltage at the battery with the engine running.

Then proceed to check the fuel pressure, O2 sensor voltage, eliminate the possiblility of the CHT sensor being bad, and check the speed and reference sensors. Make sure the idle position switch is set correctly and working. You can check the AFM (air flow meter) with a 9v battery. Bentley manual has all of these checks outlined in it.

Why did the car need a jump?
 

What is the voltage at the battery when the car runs?

Sounds like low voltage, i.e. alternator.

Thanks for the recent replies. I haven't been able to work on the car but will next weekend and will try the suggestions. To summarize what's been done to date as suggested by rusnak and outlined in the Bentley manual:
-checked the DME connections
-replaced the fuel pump relay and fuse
-visually inspected the fuel pump while running
-checked fuel system pressure
-checked volume air flow sensor output
-replaced rotor, cap, coil, and ignition wires
-replaced cylinder head temp sensor
-replaced speed sensor and reference sensor
-replaced O2 sensor
-checked luggage compartment fuses
-visually checked grounds
-replaced fuel filter
-replaced battery